It depends on what's on the picture also. I was taking pictures of last night's lunar eclipse and the file size of the images came out much smaller than usual just because most of the image was just black sky. Looks like this kind of image can be compressed a lot.

I was sending the pictures via e-mail to a friend who couldn't see the eclipse due to bad weather and the images were being compressed further. From 2160x1440 they were being reduced to 640x427 and some came out with a file size of just 11K in jpeg format.